310 •: METALLURGY OF IRON AND STEEL. the pieces quickly, as it was intended to work under normal conditions, the operation usually occupying from one to three hours. The bars were held, at the high temperature only long enough to insure uniform heating and then cooled .for several hours to about' 350° C. A longer annealing would probably have given slightly different physical results on account of the more nearly perfect elimination of strains and, transformation to cement carbon, but the difference would have been slight, and as the object was-to determine the effect of heat on the structure it was unnecessary to.. consider this phase .of the problem, • . ... Small sections were cut from the treated pieces> as well as from the untreated, and were polished and etched. They were invariably taken- from the same relative position and etched on the surface. representing the .cross section of the angle, A great majority .of these specimens when examined under the microscope showed well denned structures similar to those exhibited in Kos. 8 and 43. The orientation was apparently the same in both the'treated and the untreated bars, and the size of the grains did not appear, to be. affected by the treatment, although bars from different heats showed-considerable variation. It would therefore seem probable that as finely divided a grain ean.be produced by rolling as. by any of the usual annealing processes, although there is room for further in,-» vestigation on this point. • , • ., , • SEC. XVo.>—Theories regarding the structure, of. sieeL—Thero,• are. several theories now before the, scientific world. to> account for the hardening and the magnetic transformations in steel and' the. phenomena of the so-called critical points. It would be -better •per-:, haps to call them hypotheses, as they: are in each case offered tentatively'and as. lines: of thought on which to base experimental Te-search... It is beyond the province of this book to enter into a full -discussion of these various conceptions, but it may be well ;tp give a brief summary of the most prominent.,. / ; Tli& carbon theory considers that the effect'of hardening is due'!' entirely to a change, in the carbon contained in the, steeL In com-? mon with/the other theories, it supposes that at temperatures below; the critical point the carbon-is in the state .of cement carbon, combined with iron in the proportion Fe30. At the lower critical point. a change in carbon is supposed to occur, and since from tempera" tures above, this point carbon steels are hardened by- sudden, cooling, the advocates of* this theory have devised the name '.^hardening!